Question

1. Find values of residual enthalpy for ethylene at 400 K and 25 bar using Peng-Robinson equation of state.

2. Find values of residual Gibbs energy, enthalpy and entropy for n-heptane at 500 K and 100 bar using Virial equation of state.

3. Estimate the values of Compressibility factor, enthalpy and entropy for Benzene and cyclohexane if it compressed from 650 K and 60 bar to 500 K and 110 bar.

4. Calculate ΔU and ΔH for 1 kg of water when it is vaporized at the constant temperature of 373.15 K and the constant pressure of 101.325 kPa. The specific volumes of liquid and vapor at these conditions are 0.00104 and 1.673 m3kg-1. For this change, heat in the amount of 2256.9 kJ is added to the water.

5. Heat in the amount of 7.5 kJ is added to a close system while its internal energy decreases by 12 kJ. How much energy is transferred as work? For a process causing the same change of state but for which the work is zero, how much heat is transferred?

Answer 1

4] By 1st Law of thermodynamics,

U = Q + W....................(Q and W to the system are taken as positive)

W = -PV

.: W = - 1kg x 101.325 kPa x ( 1.673 - 0.00104) m³/kg

.: W = -169 kJ

U = Q + W

U = 2256.9 - 169 = 2087.9 kJ..................Answer

dH = dU + d(PV)

.: dH = Q -PdV + ( PdV + VdP)

.: dH = Q + VdP

As pressure is constant,

dH = Q

i.e H = Q = 2256.9 kJ

5] Q = 7.5 kJ

U = -12 kJ

By 1st Law of thermodynamics for closed systems,

U = Q + W

.: -12 = 7.5 + W

.: W = -19.5 kJ

Now W = 0 but U = -12 kJ

U = Q + W

.: -12 = Q + 0

.: Q = -12 kJ

12 kJ of heat is removed from the system.